Plasma processes are important in the aerospace, solar energy, paper, textile industries as well as in the electronics industry for the fabrication of integrated circuits and optoelectronic devices. See National Research Council, Plasma Processing of Materials, National Academy Press, Washington, D.C., 1991. For example, plasmas are used both to etch and to deposit thin film layers on integrated circuit substrates.
A plasma is an ionized gas in which concentrations of positive and negative ions are almost equal. The plasma may also contain free radicals which are electrically neutral yet highly reactive. A plasma is formed by introducing a desired gas into a reactor or chamber and applying a radio frequency (RF) field to the chamber. The gas introduced is typically chosen to participate in the chemistry of the desired process, as for example chlorine gas in etching polysilicon in the fabrication of integrated circuits. The RF field causes electron collisions with neutral or charged species which then emit radiation producing a glow-discharge or emission.
Plasma etching is the selective removal of material by the reactive free radicals or ions generated within the plasma. In many cases, the plasma etching process is superior to wet etching techniques (where material is etched by liquid chemicals) in terms of exactness of the etching and process control. See generally, R. G. Poulsen, "Plasma Etching in Integrated Circuit Manufacture -- A Review," J. Vac. Sci. Tech., Vol. 14, No. 1,266-274 (January/February 1977).
Plasma processes are generally difficult to control. See, e.g., National Research Council at 34-35. For example, the plasma etching process must be continuously monitored to compensate for variations. One cause of variation in the process is the aging of the reactor. The etch time for a freshly cleaned reactor chamber is different than the etch time for a reactor that has been in production use for a time. Also, wafers having different pattern densities etch differently. Such changes necessitate continual inspection to maintain the quality of the product. Based on the inspection results, a decision is made for the etch time for the next lot. However, the requirement for continuous human intervention to account for the effects of machine aging and cleaning leads to run-to-ran variations in wafer attributes or characteristics between lots. Thus, there is a need for an accurate control mechanism to adjust the etch times between lots without continuous human intervention.